1. Field of the Invention
This invention relates to a signal reproducing device for reproducing voice signals or the like from vector-quantized coded data.
2. Description of the Prior Art
In electronic devices which produce voice messages, voice guidance, etc. using a simple hardware configuration, a voice reproducing device performs vector pulse code modulation (VPCM) in which a low-bit code book is used for vector quantization.
FIG. 6 shows a voice reproducing device performing VPCM. In the voice reproducing device of FIG. 6, patterns stored in a code book 102 are read out as required in accordance with voice signal coded data recorded in a data ROM 101, and the thus read out patterns are sequentially converted into analog signals by a D/A converter 103. The analog signals are passed through a filter 104 for removing aliasing noise, and then amplified by an amplifier 105 to reproduce the voice information through a loudspeaker 106. The code book 102 is a byte address ROM containing 256 patterns, each pattern (representative vector) being composed of a combination of eight samples (eight bytes) of 8-bit (1-byte) data. Each pattern can be accessed using address data input to the upper 8-bit address inputs (A.sub.3 -A.sub.10) of the code book 102 (i.e. in units of eight bits). The data ROM 101 contains various combinations of the 8-bit address data of the code book 102, in the form of coded data. These address data of the data ROM 101 are sequentially read out to the code book 102 by an address counter 108 that,operates according to the output of an oscillator 107. On the other hand, the lower three bits (P.sub.0 -P.sub.2) of the count outputs of the address counter 108 are connected directly to the lower 3-bit address inputs (A.sub.0 -A.sub.2) of the code book 102. In the code book 102, therefore, each pattern is selected using the upper 8-bit address given from the data ROM 101, and eight samples of data for the selected pattern are sequentially fed in blocks of eight bits to the D/A converter 103 in accordance with the lower three bits (P.sub.0 -P.sub.2) of the output of the address counter 108.
As a result, in the above voice signal reproducing device utilizing VPCM, a voice signal of 64 bits (8 bits.times.8 samples) can be compressed to 8-bit address data for storage in the data ROM 101.
The coded data contained in the data ROM are previously generated by vector quantization in which patterns closest to the voice signals to be coded are sequentially selected from the above-mentioned code book. When encoding the voice signals, therefore, all the patterns in the code book need to be sequentially read out for comparison for every eight samples of each voice signal. In contrast, when reproducing the voice signals, realtime processing can be achieved easily by random-accessing the code book using the coded data.
However, the above prior art voice signal reproducing device utilizing VPCM has a problem in that the code book 102 requires a large capacity for storing a number of patterns (in the above example, 16 kilobits (=8 bits.times.8 samples.times.256 patterns)). This memory capacity requirement is still too severe to make the hardware configuration simple enough. Furthermore, the plurality of patterns stored in the code book 102 must be selected on the basis of the actually used voice data so that the quantization error distribution in the vector quantization can be minimized. As a result, these patterns are dependent on the actually used voice data, and when the voice data are changed, the patterns must be reselected to match the new voice data. This impairs the versatility of the ROM that constitutes the code book 102, preventing the reduction of costs by mass production.